This invention relates to communication systems with multi-tone transformation and, in particular, communication systems in which the multi-tone transformed signals are phase shifted in order to reduce their peak to average power ratios.
Communication systems with multi-tone transformation are well known in the art. The multi-tone transformation is also referred as a discrete multi-carrier modulation or orthogonal frequency division multiplexing (OFDM).
Multi-tone transformation has widespread practical communication applications including communication systems using twisted pairs of wires which provide high speed wireline communication connections. Further, its hardware implementation lends itself to discrete Fourier Transformation (DFT) structures which are both inexpensive and well understood.
FIG. 1 illustrates a conventional multi-tone transformation transmitter 101 which includes a channel coder 103, a constellation mapper 105, an Inverse Discrete Fourier Transform (IDFT) device 107 and an RF amplifier 109. Illustratively, the output of RF amplifier 109 is provided on a pair of twisted wires 111 or over a wireless communication channel. The channel coder 103 receives a plurality of input bits. The combination of the channel coder 103 and the constellation mapper 105 converts the received input bits to constellation symbols. The constellation symbols are input to the IDFT device 107, and its output signal is fed to the RF amplifier 109. With certain combinations of the constellation symbols, the output signal from IDFT device 107 has peak signals having larger signal magnitudes than that of average signals. This requires that the RF amplifier 109 operate linearly for a wide range of magnitudes in order to avoid clipping the peak signals. Amplifiers that have a wide dynamic range are expensive. Furthermore, practical amplifiers with reasonable dynamic range saturate at a certain peak and may attenuate the peak signals. This is known as the peak to average signal power ratio problem.
A number of possible solutions to the peak to average signal power ratio problem have been suggested. For example, U.S. Pat. No. 5,835,536 (May), which is incorporated herein by reference, teaches a communication system which generates time-domain discrete multi-tone symbols. Its peak to average power requirements are reduced by (1) comparing the magnitude of the time-domain discrete multi-tone symbols with a magnitude threshold and (2) clipping the magnitude of those time-domain discrete multi-tone symbols which have larger magnitudes than that of the magnitude threshold. However, by clipping the magnitudes, it loses information.
In another example, U.S. Pat. No. 5,838,732 (Carney) discloses a multi-channel wireless communication system which produces digital baseband modulated signals by modulating digital baseband channel signals according to its desired interface standard. In order to reduce its peak to average variance, the digital baseband modulated signals are then phase shifted by digital phase shifters.
The present invention provides a simplified and efficient method and system for reducing the peak to average power ratio (PAPR) in communication systems having multi-tone transformation. This is achieved without changing the error correction capability of the overall system, the encoding rate of its transmitter, and the decoding complexity of its receiver.
The method of the present invention includes the steps of receiving at least one block of bits of information and then generating a codeword which includes an N number of digital multi-tone symbols based on the block of input bits, wherein N is a positive integer. The method further includes the steps of calculating a plurality of phase shift values for the digital multi-tone symbols that minimizes the PAPR and phase shifting the digital multi-tone symbols by the phase shift values, to thereby reduce the PAPR.
The system of the present invention includes an encoder configured to receive a plurality of input bits and to generate a plurality of coded symbols based on the plurality of input bits and a phase shift calculator configured to calculate a plurality of phase shift values for the coded symbols that minimizes the peak to average power ratio. The system further includes an encoding phase shifter coupled to the encoder and the phase shift calculator, configured to receive the phase shift values from the phase shift calculator and configured to phase shift the plurality of coded symbols to generate a plurality of phase shifted symbols and an amplifier coupled to the encoding phase shifter and configured to amplify a plurality of symbols it receives.